High-oil things do seem to want to leak and I have had similar problems with olive oil, pesto, ghee / clarified butter, even chocolate when it melts. It is definitely worth extra effort at careful packaging on all these since they make awful messes when they leak. This is, obviously, a bigger problem when the expected weather is warmer.
If you want to keep it simple, peanut butter and nutella jars work pretty well, but can't handle the heat of a water-bath sterilization. I'd probably carry them in a freezer-grade baggie to minimize the damage of any slow leak.
But, there are ways that you can have pretty good seals and a jar that can handle higher heat.
I have had pretty good luck with 8-oz. wide-mouth brand-name Nalgene bottles. I've found that when I tighten the tops I get a better seal if I first rotate the top counter-clockwise until I am sure that the threads are meshing properly and then tighten pretty hard in the usual righty-tighty (clockwise) direction. If you are not very careful, you can get the threads slightly mis-aligned and not get a good seal.
The square wide-mouth Nalgenes look like this (there are also round ones)
Link for picture:
There are nice 16-oz wide-mouth jars that look like Nalgenes but actually made by Vestil which I have found less god than the brand-name Nalgenes for things like olive oil. (Nalgene has some patented leak-proof cap design.) I usually will carry my Vestil jar in a freezer-grade baggie in order to contain any slow leak. Next time I am in the market for a 16-oz jar I would probably look for a brand-name Nalgene, but I can live with the Vestil jar and a baggie for now. The leak is usually slight and contained by the baggie
The Vestil wide-mouth 16-oz jar looks like this and I use it often for olive oil. I like the measurement scale on the side so I can use the right amount each day without getting a measuring cup all oily. I get them from Grainger.com
Link for picture:
But for pesto or clarified butter, I prefer jars to wide-mouth bottles because it is easier to get the stuff out..
PP (Polypropylene - Recycle Number 5) plastic jars from USplastic.com have worked well for me but only if I am careful about the liner that goes inside the cap. The jars are PP (Polypropylene) and handle heat well. The lids are also
Polypropylene and can handle heat. But the liners (F217 - Foamed polyethylene) degrade with heat. So I remove the liner from the cap and then sterilize the jar and cap with heat (at least 85 degrees C for at least five minutes - but hotter and longer are better). I use alcohol (rather than heat) on the F217 liner and try to put the liner back into the cap carefully with my hands as clean as possible. Clearly this is not truly sterile but it is the best I can do without degrading the cap liner.
The properties of these materials are described here:
The model numbers at US Plastics that have worked for me for jars are 70292, 70293 and 70294 for 8-, 12- and 16-oz jars. Caps are ordered separately and are model numbers 66386 and 66387. Make sure you order the right cap size for the jar - the website will specify 70- or 89-mm caps as needed - for my jars the 8-oz uses the 70mm cap 66386 and the 12- and 16-oz use 89mm cap 66387.
Jar looks like this (8, 12 and 16-oz all look similar)
Link to picture of jar
Cap looks like this:
Link to picture of cap
For blocks of chocolate, my current plan is to re-use one of the foil-like bags that Starbucks uses to package ground coffee. It wouldn't hold oil, obviously, but should contain melted chocolate pretty well.
Link for picture:
Your friendly research librarian,
John Curran Ladd
1616 Castro Street
San Francisco, CA 94114-3707